I would say yourself? Spam emails are from websites that you signed up with. Right? How else can someone get your emails? I hardly get any spam on my GMAIL account cause I only use it for legit websites.
You don't get it because GMail has an amazing Spam filter. You don't have to sign up for anything, just have a "normal" e-mail address and your e-mail address will receive Spam even if GMail filters it for you.
That and "hardly" any spam is different from having your iPhone, iPad and computer all go off at the same time with someone's Spam message. Gmail spam is easy to ignore, phone spam is harder.
Also, they don't need your Apple ID, iMessage can send to phone numbers too:
One problem we noticed was that sending an iMessage to an iPhones phone number meant the message didn’t appear on the Mac – and vice versa – sending an iMessage to the email address didn’t appear on the iPhone…
http://gourmetpixel.com/blog_wordpress/?p=85
While I, personally, might not be able to get a specific individuals Apple ID, I sure as hell could come up with a few ten thousand legitimate ones just by using existing Spam e-mail lists, stripping the @... and replacing it with standards such as @gmail, @me, @mac, @hotmail, etc.
Once again. What is protecting my iMessage account other than obscurity of my Apple ID?
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Right now, if someone wants to send a text message from my phone they have to pay for the text message service and are relatively traceable. That is gone with iMessage for OS X. Little Cost overhead and lots of exposure i.e. your phone actively alerting you to the message, your iPad alerting you, and your computer alerting you all at the same time and all with a pop-up window.
It is annoying enough when I get the occasional text message spam, I can't imagine how annoying it would be if it was on the same level as e-mail spam.
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http://reviews.cnet.com/8301-19512_...-block-text-message-spam-on-your-iphone-at-t/
I wonder if iMessage completes messages sent to:
yourmobilenumber@txt.att.ne
Now they don't need your Apple ID. Just a list of the block of phone numbers that AT&T has.
The article did have good information on how to block those annoying e-mail spams I was getting though.